As this type of apparatus, there is an apparatus using an exhaust gas system which is provided with a first air-fuel ratio sensor without a catalyst layer and a second air-fuel ratio sensor with a catalyst layer on the upstream of a catalyst disposed in an exhaust system (refer to Patent Literature 1). According to an apparatus which detects an abnormality of a variation of air-fuel ratio among cylinders disclosed in the Patent Literature 1, in the exhaust gas system, a shift of the output value of the first air-fuel ratio sensor to a rich air-fuel ratio side due to hydrogen caused by a variation of the air-fuel ratio among cylinders (i.e. the air-fuel ratio imbalance) is determined on the basis of difference between the outputs of the two sensors. It is therefore considered that the abnormality of the variation of the air-fuel ratio among cylinders is accurately detected with little influence of noise.
Moreover, the Patent Literature also discloses such a configuration that the output of the first air-fuel ratio sensor is corrected on the basis of output peaks of the first air-fuel ratio sensor and the second air-fuel ratio sensor.
There is also proposed such an apparatus which is provided with sensors configured to obtain air-fuel ratios on the upstream and downstream of a catalyst and which is configured to set a correction amount on the basis of a difference between a downstream air-fuel ratio target and a sensor output while allowing that a F/B correction amount by the downstream sensor is beyond a guard range (e.g. refer to Patent Literature 2).